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Selecting Fibre Optic Cables | Avnet Abacus

Selecting fibre optic cables

Selecting the right cable and/or connector assembly for a fibre optic installation needn’t be complex, but it is important to understand the requirements of the environment and the application. Different transmission standards and fibre types must also be considered. Here’s our guide to selecting the right cable for the job.

Requirements of the Application

Clearly, different products and set ups will be required for a fibre optic installation inside an office building compared to an outdoor installation linking multiple buildings, or even a sub-sea cable linking more than one country. Consider carefully the requirements of the environment such as the likely ambient temperature range, the possibility of moisture ingress and whether the cable assembly will need to withstand loading or vibration. Since glass is obviously fragile, the fibres can be cladded with a variety of different materials to improve their tensile strength.

Generally, bundles of fibres will then have an outside jacket to protect them from the environment; outdoor cables will be waterproofed with either a gel filling or a dry powder or tape. Some cables that are intended to be buried will have a metal jacket to prevent rodent attacks. Some are even designed for both indoor and outdoor use. There are a wide variety of protection and jacket options available, and for systems that require more than 1km of cable, bespoke protection offerings may be considered.

Other factors defined by the application include the space available for the cable and connectors, and the cost involved, while increasing requirements for more data bandwidth are leading to adoption of fibre optics cabling in areas such as industrial, residential, transportation and medical equipment. A wide range of cable housing materials are available along with certifications where required – for example certification to UL Standards for industrial applications.

Types of Fibre

There are two main types of fibre optic cable: single mode and multimode.

Multimode fibre allows light to travel down the fibre in many different pathways. It is used in inexpensive systems which use LEDs as light sources for data rates of 10 to 100 Mbit/s, and comes in core diameters of 62.5 and 50 µm (called OM1 and OM2/3/4, respectively). The 50 µm type is usually chosen nowadays because it offers up to 20x the bandwidth of the 62.5 µm type (OM4 is the highest bandwidth, OM3 is medium, OM2 is the lowest).

Single mode fibre is used with more expensive VCSEL (vertical cavity surface-emitting laser) systems which can achieve speeds up to 10 Gbit/s. The VCSEL laser focuses the light to a narrower band inside the specially optimised glass fibres, which allows the signal to travel further, which is useful for TV and telephone systems. Single mode fibres generally come with an 8.3 µm diameter core.
Selecting a cable with more bandwidth than you need maybe a sensible choice if you anticipate upgrading the network in the future. Also, it is prudent to include a number of spare fibres in case any break during installation, since in general, it costs less to specify extra fibre than it would to install new cables.

While glass optical fibres are still used in telecoms and high performance applications, for low-speed applications, plastic optical fibre (POF) is available. The core size is much larger than for glass and the speeds and distances achievable are much lower, but the overall system cost is very low by comparison because the POF’s robustness to bending makes installation much easier. When compared with copper cabling, POF benefits from its lightweight construction and signal immunity from EMI/EMC interference.

Today’s optical fibre cables come in two formats - simplex (single fibre for one way comms or multiplexed transmit and receive) and duplex (two fibres, one for transmit and one for receive). Duplex fibres are usually in ‘zipcord’ configuration, which means they are side by side (cross section like a figure 8).

A leading supplier of fibre optic components, Molex has an extensive range of solutions for optical connectors and adapters, optical assemblies, circuitry, termination and tooling for several end markets including demanding industrial, military and medical applications. An online POF Cable Configurator makes it easy to begin evaluating a customised solution by specifying the key requirements for the connector and cabling solution.

Types of Connector

The most popular connector type is SC, a 2.5-mm connector (right) which is pushed together and clicks when seated, and ST, which is a bayonet style twist-lock connector for 2.5-mm multimode fibre. However, many more types are available. FC, a 2.5-mm screw connector is sometimes used for datacom and telecoms equipment. LC is a 1.25-mm snap connector used in high density connections, and MTRJ is another popular snap connector for duplex multimode cables.

For assistance selecting optical fibre cables, connectors and assemblies, get in touch with our team of technical specialists in your local language by clicking the Ask an Expert button to the right of this post.

Written by

Marco Enge

As a Senior Product Manager Marco is responsible for product marketing and strategy for interconnect solutions. He has over 20 years experience in electronics having begun his career with Siemens, and occupied roles with Vogt Electronic and Sun Microsystems before joining Avnet Abacus in 2006.

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